Remember that this key grants access to all of your portal’s data, so keep it secret. If you’re sending form submission data, you might also want to check out the HubSpot Forms API, which doesn’t require authentication to submit form data - http://developers.hubspot.com/docs/methods/forms/submit_form

This API endpoint will be utilized by a custom integration that you would build. You would fire an HTTP Post request when you wanted to create or update a contact in HubSpot from your 3rd party system. The documentation linked above will show how to properly send data to this endpoint.